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[CrappieMagnet]

Explanation of how barometric pressure affects fishing
by Dr. David A. Ross

Does a changing barometer truly affect our fishing success? Let science answer that question.

FISHERMEN SOMETIMES HAVE ideas or opinions about the marine environment that do not stand up to scientific scrutiny. For example, many anglers believe that changes in barometric pressure strongly influence fish behavior—most notably their willingness to cooperate with anglers. Some have even written that fish can detect a change in barometric pressure before it occurs. An interesting notion, perhaps, though in almost all instances it is incorrect.

A rise or fall in barometric pressure, such as with an approaching cold front, usually means a shift in the weather pattern. And it is the change in the weather, not any fluctuation in barometric pressure, that affects both the fish and the fishing. In fact, most saltwater species probably aren’t even aware of barometric variations.

Pressure, whether in the air or in the ocean, is expressed by scientists as units of “atmosphere.” One atmosphere is defined as the pressure caused by the weight of all the overlying air at sea level—or 14.7 pounds per square inch (psi). Atmospheric pressure is often called barometric pressure because it can be measured by the height of the mercury column in a barometer. Changes in barometric pressure, therefore, indicate capricious weather. In general, low-pressure systems bring unstable conditions, often with precipitation and clouds. A rising barometer means high-pressure is approaching, the harbinger of stable and clear skies.

How much do fish respond to these day-to-day fluctuations? Consider that a normal value for barometric pressure is about 30 inches. Strong high pressure is about 30.70 inches. A powerful low, such as during a hurricane, can reach down to 28 inches or less. The difference between these two extremes (2.7 inches of barometric pressure) is equal to about .09 atmospheres. The barometric pressure difference from a simple passing cold front is only about .06 atmospheres.

The rate of a falling barometer also tells us how fast a low-pressure storm is approaching. A slow-moving storm would have a dip of about .02 to .03 inches of barometric pressure per hour; a fast-moving storm will drop the barometer about 0.05 to 0.06 inches per hour.

Simply stated, barometric pressure does not change quickly enough to magically turn the bite on or off. It certainly is one of the ingredients in the overall weather process, but temperature, cloud cover, wind direction and speed, and humidity can also affect fishing conditions. More importantly, the rate and amount of change in barometric pressure is insignificant compared to what’s going on below the surface.

Beneath The Squeeze Pressure in the ocean, called hydrostatic pressure, increases with depth due to the weight of the overlying water. Water is almost 800 times denser than air; thus, hydrostatic pressure increases much more rapidly than atmospheric pressure. If you swim or dive just a few feet below the water’s surface, you feel this rapid increase in pressure.

At a depth of just 32.8 feet in the ocean, the hydrostatic pressure is equal to the pressure from the entire weight of the earth’s atmosphere as measured in pounds per square inch. In other words, at 32.8 feet, the total pressure, due to the weight of both the atmosphere and the water, is two atmospheres. At 65.6 feet it’s 3 atmospheres, and so forth.

Fish can tolerate hydrostatic pressure because they have a swim bladder containing a volume of gas, which they adjust to equal their environment. This enables most fish to comfortably make small and quick up or down movements in the water column.
[Barometric pressure]

In the ocean, four main factors can change the hydrostatic pressure in the fish’s world. First, a fish naturally changes pressure around itself by making movements associated with feeding, swimming about, avoiding predators or trying to loose a hook. A small move can result in a relatively large pressure variation. For example, going up or down just 3.28 feet will decrease or increase the pressure on a fish by 1/10 of an atmosphere. One tenth of an atmosphere exceeds any reasonable change that might occur due to a fluctuation in barometric pressure. Equally important, when barometric pressure rises or falls, it can take more than a day to equal the change in hydrostatic pressure that a fish experiences in seconds during its normal up or down movements.

Second, tides can alter hydrostatic pressure. Assuming the fish stays in the same position, even a small three-foot rise in tide will increase the hydrostatic pressure by about 0.09 atmospheres. A low tide would decrease the hydrostatic pressure by a similar amount. Thus, within about a six-hour period from high to low tide, a fish would experience a fall of about .18 atmospheres of pressure. This is about twice what could be expected from the barometric pressure going through a major drop during a hurricane.

Third, waves make rapid and continuous changes in hydrostatic pressure. Two-foot waves, for example, will produce a change in pressure of about .06 atmospheres. This rapid change correlates to the period of the waves—about four to six seconds. Higher pressure comes when the crest passes; lower pressure occurs under the trough. When a storm approaches a coastal area, the waves, and the increase in hydrostatic pressure, will be considerably higher than during calm-weather periods.

The weight of the air itself is the fourth influence on hydrostatic pressure, but its effect is quite gradual. Barometric pressure associated with a major storm will dip (depending on the system’s rate of speed) by only .002 to .02 atmospheres per hour. This gives fish considerable time to make any necessary adjustments. When compared to the effects of the tide, waves, and normal movements of the fish in the water column, changes in hydrostatic pressure caused by barometric-pressure are trivial for saltwater fish. Even a dramatic change in the barometer will be lost to the everyday pressure changes experienced by fish under normal oceanographic conditions.

It’s a happy notion that one could simply consult the mercury column each morning to know whether it’s a better day for work or fishing, but it’s unlikely that barometric pressure alone can trigger the sudden bite that angling’s common wisdom often asserts.
Dr. David Ross is a scientist emeritus at the Woods Hole Oceanographic Institution and the author of The Fisherman’s Ocean (Stackpole Books). He is also a regular columnist for Saltwater Fly Fishing Magazine. He can be contacted at [email protected]. This article first appeared in Saltwater Fly Fishing Magazine. © 2004 Dr. David A. Ross and Saltwater Fly Fishing Magazine.

[Tunica]

Ok but the biggest thing that effect the fish on those lakes I fish on is ME...If I'm fishing they dont bite the minute I trailer and leave the bite is ON.

[Barnacle Bill]

I can't help but wonder how many times this guy has gotten out behind his desk and went fishing.

[deputyking]

Yes, I will also believe the fishermen who fishes everyday and has not only studied the barametric pressure effects on fish but who also has wittnesed it day in and day out. I myself believe that fish have the most sensitive pressure gauge and they may not be affected so much as what the pressure is doing to them but what the pressure is going to do to the weather and responds accordingly. There are many factors that affect the feeding or bitting of fish and this is only one of them. Throwing an anchor in the middle of bedding fish will also have an effect on their feeding even if other factors are favorable. I believe that they sometimes can even tell when my trailer enters the lake to unload my boat and stop bitting.

[skiptomylu]

I can't help but wonder how many times this guy has gotten out behind his desk and went fishing.

Ditto, clearly he has never been fishing the day before a cold front and then the day after the same cold front. If anyone has ever fished before and after a front I do believe they have seen the difference it can and does make.
Attachment 5579

[CrappieMagnet]

Oooops....now let me post the rest...MY BAD.


No wonder you all were saying something....LOL
I'm gonna go hide behind the smallest rock.LOL

[izzysfishin]

I don't know nothing about atmospheres and Hydrostatic pressure is what scientists are full of. There is'nt a fisherman or hunter that i know, will dissagree with the FACT that fish and game will put on the feed bag before a big weather change. I could'nt tell you why if my life depended on it. If i can get in the woods or on the water before the front comes through, i'm there...izzy

[dmann436]

and analyze things to the inth degree on paper all that bull corn might make sense BUT if you've fished very many years you know what to expect when a front moves in an out and you don't need the barometric pressure to tell you whats going on--right fellers? how many yall check the pressure before you go fishin and decide to go or not just by what the barometer reads??
and ditto Izzy

[Ferdi]

Just wondering, is it the weather changing that makes the barometer change, or is the change of the barometer what makes the weather change? Y'all got me totally confused. Anyway you look at it the fishes know when something outside the water is happening and they either put on the feed bag or they clam up. I'm usually there for the "CLAM UP" session. Ferdi

[Litebite]

It has been known for a long time that the barometric pressure has an effect on fishing. How the pressure directly effects the fish is still not fully understood, but knowing how to use the barometric pressure readings can greatly increase your chances of catching fish, especially in shallow and fresh waters.

Barometric pressure is the measure of the weight of the atmosphere above us. It exerts pressure on the waters we fish and even on us. In fact, it can change how well some people feel. It is believed by many, that it may have a similar and even more dramatic effect on fish effecting their feeding habits.

Measurement of barometric pressure is accomplished with the use of a barometer. A barometer measures the weight of the atmosphere per square inch (pressure) and compares it to the weight of a column of mercury.

The first instrument was invented in 1643 by Evangelista Torricelli. His barometer used a glass tube from which all air has been removed (a vacuum) and is inserted into a container of mercury that is exposed to the pressure of the air. The air pressing down on the mercury in the container forces an amount of the mercury up into the glass tube. The height to which the mercury rises is directly proportional to the pressure of the atmosphere. This is usually measured in inches (inHg) or in millibars (1 inHg equals 33.864 millibars).

Today aneroid barometers, invented by the French scientist Lucien Vidie in 1843, are the most widely used instrument to detect air pressure. An aneroid is a flexible metal bellow that has been sealed after removing some of it's air (a partial vacuum). A higher atmospheric pressures will squeeze the metal bellow while a lower pressure will allow it to expand. This expansion of the metal is usually mechanically coupled to a dial needle which will point to a scale indicating the barometric pressure.

A new form of barometer uses a pressure transducer. This transducer is like a miniature aneroid barometer that converts the amount of air pressure into a proportional electrical voltage. This voltage then can be fed into a digital readout and/or into a computer.

Barometric pressure varies with altitude. A higher elevation will have less atmosphere above it which exerts less pressure. To keep readings standard across the world, barometric pressure is to be indicated at sea level. Therefore, readings at elevations other than at sea level will require a correction factor which is based on the elevation and the air temperature (colder air weighs more and will require a greater correction).

The barometric pressure changes as the weather systems over us changes. When you look at a weather map that has those blue "H"s and red "L"s, this is indicating the areas with High and Low pressure. It is worth noting that the areas with high pressure are the areas with good weather, and the areas with low pressure are the areas with bad weather. Barometric pressure has been used by weathermen since the beginning of meteorology to predict the weather. It can also be used by fishermen to predict the quality of fishing, and more importantly, how to fish.

As a general guideline, think of 30 inHg (1016 millibar) as being a normal level. World records vary from a high pressure of 32.0 inHg in Siberia to 25.7 inHg during a typhoon (both readings are off the scale of most barometers). For the US, extreme levels can be considered as 30.5 inHg and 28.5 inHg. When it comes to fishing, a change of just +/- 0.02 inHg from normal is enough to effect their feeding habits.

It is important, however, to note that the effects of barometric pressure is greater in fresh and shallow waters, than it is in deeper waters. This is probably due to the fact that the pressure of water is so much greater in deeper waters making the air pressure above it no longer having any significance.

Some general rules regarding barometric pressure are:

1. When the pressure trend is high, the typical weather will call for clear skies. Fish slow down, find cover or go to deeper waters. Slow down lures and use baits more attractive to fish. Fish in cover and in deeper waters.

2. When the pressure trend is rising, the typical weather will be clearing or improving. Fish tend to become slightly more active. Fish with brighter lures and near cover. Also fish at intermediate and deeper depths.

3. When the pressure is normal and stable, the typical weather will be fair. The fishing trend will be normal. Experiment with your favorite baits and lures.

4. When the pressure begins to fall, the weather will begin degrading. During this perios the fish become more active. Speed up your lures. Surface and shallow water lures may work well.

5. When the pressure becomes slightly lower, the skies will usually become cloudy. Many fish will head away from cover and seek shallower waters. Some fish will become more aggressive. Use shallow running lures at a moderate speed.

6. When the pressure trend becomes low, the weather will most likely be rainy and stormy. Fish will tend to become less active the longer this period remains. As the action subsides, try fishing at deeper depths.

It is important to note that after a long feeding period, the action will slow regardless of the following conditions. On the flip side, a long period of poor fishing conditions may be followed by a really good one.

It is also important to note, that the barometric pressure is just one of many factors that effect fish feeding habits. Other effects include water temperature, light, tidal forces, water clarity, the pH level, water levels, wind/surface disturbance, boat traffic, fishing pressure, and so on. Another good judging factor of fishing is the solunar effects which play a role in the tidal and illumination factors.